I don't know whether you are doing something wrong, but when I plotted the cubic polynome the root did not look pretty either.View attachment 29779
The equations I got from setting the two partials equal to zero are:
y = 0.5 + 0.2x^2
x^2 = (4y + 4y^3)/5
When I try to solve for these I keep getting crazy decimals so I feel like I'm doing something wrong